1. Field
Embodiments of this invention relate generally to a nonvolatile semiconductor memory device.
2. Background Art
A method for collectively processing a three-dimensional multilayer memory has been proposed to increase the memory capacity of a nonvolatile semiconductor memory device (memory) (see, e.g., JP-A 2007-266143 (Kokai)). In this method, a multilayer memory can be collectively formed irrespective of the number of stacked layers, and hence the increase of cost can be suppressed.
In this collectively processed three-dimensional multilayer memory, insulating films and electrode films (to serve as word lines) are alternately stacked to form a multilayer body, in which through holes are collectively provided. Then, a charge storage layer (memory layer) is provided on the side surface of the through hole, and silicon is filled inside the through hole to form a silicon pillar. A tunnel insulating film is provided between the charge storage layer and the silicon pillar, and a block insulating film is provided between the charge storage layer and the electrode film. Thus, a memory cell illustratively made of a MONOS (metal oxide nitride oxide semiconductor) transistor is formed at the intersection between each of the electrode films and the silicon pillar.
In such a collectively processed three-dimensional multilayer memory, further stabilization of its erase state has many advantages, such as higher controllability in the write operation.
A method of performing a plurality of erase loops as the erase operation has been proposed for a three-dimensional multilayer memory with planar memory cells simply stacked therein, not for the collectively processed three-dimensional multilayer memory as described above. However, because the structure and operating mechanism of memory cells are different from those in the collectively processed three-dimensional multilayer memory, this method is not directly applicable to the collectively processed three-dimensional multilayer memory. Thus, it is necessary to develop an operation method specific to the collectively processed three-dimensional multilayer memory.